Absorption of Hydrogen Isotopes by Vanadium-based Low Activation Materials

钒合金是聚变堆低活性结构材料的潜在候选者。然而，包括氚在内的氢同位素在这些合金中的渗透率很高。众所周知，氢同位素通过第5族金属的渗透敏感地依赖于表面状态，但对这些合金还没有进行系统的研究。从这一角度出发，结合X射线光电子能谱表面分析，研究了V-Ti和V-Ti-Cr合金对H_2和D_2的吸附，以了解表面化学状态与氢同位素反应速率之间的关系。将由美国国家核聚变科学研究所提供的V-4Ti合金和V-4Ti-4Cr合金的片式样品安装在超高真空室中，并对其在673-1273K热处理后的表面进行了分析，在800K以上，两种合金的表面都明显地观察到了钛的富集化。用不同的热处理方法制备了不同表面钛含量的样品，并在523~1023K温度范围内进行了吸氢性能测试，未富钛的样品表面的附着系数α为10^&lt；-3&gt；-10^&lt；-4&gt；，而钛的表面偏析达到[Ti]/[V]~0.5时，α降低了2-3个数量级。也就是说，钛的表面偏析导致了阻碍氢渗透的阻挡效应。用从头算方法估算的氢解离吸附势垒表明，钛的直接影响可以忽略不计。结果表明，α的降低是由于钛的存在增加了杂质氧的表面覆盖度，而钛与氧的化学亲和力大于钒。在目前的条件下，氢与氧之间的同位素效应很小，不能观察到。

Vanadium alloys are potential candidates of low activation structural materials for fusion reactors. Permeability of hydrogen isotopes including tritium, however, is high in these alloys. It is known that the permeation of hydrogen isotopes though group 5 metals sensitively depends on surface states, but the systematic study has not been carried out for these alloys. From this viewpoint, absorption of H_2 and D_2 by V-Ti and V-Ti-Cr alloys was examined in the present study in combination with surface analysis by X-ray photoelectron spectroscopy to understand the correlation between surface chemical states and the reaction rates of hydrogen isotopes.Sheet type specimens of V-4Ti alloy and V-4Ti-4Cr alloy supplied by the National Institute for Fusion Science were installed in an ultra-high vacuum chamber, and their surfaces were analyzed after heat treatments at 673-1273 K. Enrichment of Ti on the surfaces were clearly observed above 800 K for both alloys. The specimens with different surface Ti concentrations were prepared by respective heat treatments, and hydrogen absorption was examined in a temperature range from 523 to 1023 K. The sticking coefficient α on the surface without Ti enrichment was 10^<-3> -10^<-4>, while the surface segregation of Ti up to [Ti]/[V] 〜0.5 led to the reduction in α by 2-3 orders of magnitude. Namely, the surface segregation of Ti caused the barrier effect against hydrogen permeation. The potential barrier against dissociative adsorption of hydrogen evaluated by ab initio calculations showed that direct influence of Ti is negligibly small. It was concluded that the reduction in α was due to the increase in the surface coverage of impurity oxygen caused by the presence of Ti whose chemical affinity to oxygen is larger that of V. The isotope effect between H_2 and D_2 was small and not observable under the present conditions.